Carrier-current telegraph system



v. P. 'rHoRP.'

G'ARRIERv CURRENT TELEGRAPH SYSTEI Aug; 2s, 1930,!

A 'Filed Aug'. .10, 19,28

Osclator J2 www INVENTORy BY WW ATTORNEY.

0 "method, i; e.,icarrier current-is transmitted Patented Aug. 26, 1930.'Y

VAUGHN P.' Timur, fornrvnnfnnen, NEW" massif',v ssrsivon' AMERiCAN TELE# Y f ,rHoNEiAND TELEGRAPHcoMrANY, A coRronA'rroN or NEW vom;

.jappncatiqn mea A'August io,

,- 'Thisinventi'on,relatesto the art of tele- `graph communication and discloses va car- -riercurrenttelegraph system in which distinct yfrequencies jareu-tgilizedfor the transmission of the marking and spacing frequencies. n I. .l Y l lAtgthe present time; it is the. general ypractice in carrier current telegraphyto transmit signals Vhy the' socalledopenand-close over thefline during the marking interval whereas no current is transmitted during-the `spacing interval. At the,receivingfterminus of thecarrier line-fthe; rectified carrier `signais constitute pulses of; direct current suc-V ceeded by-intervals ofnn'oci'u-rent-.' A Signals sitive polar relaywhich is biased toward4 the spacingicont-act. y hy means-of 'a ,current through'anfauxiliary Winding.- j I The open-and-close method ofr operation discussed y immediatelygabove,`v has a serious disadvantage in thatchangesfin line attenuation cause fthe-s1:gnals to become Tbiase'df yFor example, `if the,v constant biasing current through' the yauxiliary winding of thev polar relay mso-'adjusted that there' is no signal Some; signal ,bia's, the rmagnitude.. of which change. f many `factorsWhlch causervariatlonsfin lineV ,Y will' he y dependentupon the vamount of such lngo'perating practice, thereare attenuationfsuch, lfor example, as theieiect of ,temperature variations on, the conductor resistance .or 1in:` the` case of openwwiretf lines the enormous variatlonin line: leakage With humidity conditions. Withtheahove methodv of openv and ycloseoperation,fzeach changeset line attenuation v(jf/any,considerable mag-` nitude Wu'ldlreq'uirea ireadju'stment of the constant Vbiasing current or readjustment ot' rthe gain of 'the .receiving amplifier.'`

t'llhefpresent lnventionghas been developed VWith'v the view tovvard providing'what in *l effect apelar lmethod of operation; and Vone 'andclose operation discussedabove. e'

'frThe 'circuit operation-is' best understood .Which-is'not opento-the drawbaeksencountered-in connection with th'efmethodofopen v reference tozthe drawing Whichshows in i 'i though it isf understoodVof course,-that hy means of simple circuit transformationsfWell lranged for. half duplex operation.

knownjto the art,`\the' circuit could be ar` 'Referring to the draw ratone, terminus ofthe carrier linee-tlf VThe i sending' circuit includes sendingy key 22 and telegraphfsounder 3l located at asubscribers station and connected over'jloop 24L torthe carrier sending;V means Whichi'fincludes:polar relays letKV '50165;inclusivak soiiirce of carrier current 26, amplifier 25, and af connection ,through certaingtuned circuitstocarLr-ier line ljthrourqfh transformer 2. #Tliegreceiving ciry cuit'sliovvn 1in th'euppejr, portion of thedraW#VV ing .isals'o connected to="the carrierflinefl throughtranstormer 2, and comprises certa-iu" otherl tuned'y circuits, amplifier-,detector polarrelaysto 6, inclusive,receiving loop 23, and telegraph sounder 2lalocated ata `sub-i` kThevsu'lostation equipment shoivnis adapt-f ed for 1trans'mission.in accordance with the AMorse -codeV but fityi's to VVloe understood, ot F l conrseythat 'other substation equipment such ,asprintingtelegraphsets might be used Witli outv'aftecti'ngthe novel features of the in-l vention. y

. vl-eferringfto thefsfending circuit, itvvill he I Y notedt'hat with key 22 down asshowmcurover, thelowerfconductor oijloop 245 through Vren'tf'tlowsfrom grounded plus' battery 30, v

no f Wimax W i i t i sounder 31, key 22, back over the upper conductor of loop 24, through resistance 33 and through the line or upper windings of polar relays 14 to 16, inclusive, in series, through minus battery 29 to ground. This current holds the polar relays in question operated against their respective marking contacts M. In addition, current fiows from grounded negative battery 29 through the lower or auxiliary windings of relays 14 to 16, in-

f elusive. in series. This current tends to opcrate the relays in question against their respective spacing contacts S but being of smaller magnitude than the current in the line windings will not so operate the relays while the above-described current iiows in the line windings. If, however, the current in the line windings of the relays is interrupted as by openingV key 22, the current through the auxiliary windings immediately operates the relays against their respective spacing contacts.

It will thus be seen that polar relays 14 to 16,inclusive, follow the operations of the sending key 22. With relay 16 operated against its marking Contact M, condensers 27 and 28in parallel are shunted across the tuning inductance associated with oscillator 26, thus causing the oscillator to oscillate at a definite frequency f1. lVit-h relays 14 and 15 likewise operated against their marking contacts M, condensers 17 and 18 in parallel are in series with the left winding of transformer 32 and condensers 19 and 20 in parallel are shunted across the right winding of'transformer 32. These capacities associated with the inductance of the transformer windings in the manner shown constitute a pair of tuned circuits adapted to transmit the marking frequency f1 generated by oscillator 26. The result is that frequency f1 generated by oscillator 26 flows through amplifier 25, through transformer 32, and out over carrier line 1 through transformer 2 to the distant station.

If, now, the sending key 22 be opened, relays 14 to 16, inclusive, will move against their respective spacing contacts S, as explained above. The operation of relay 16 against its spacing contact will open the circuit containing condenser 28 and will thus leave only condenser 27 connected to the tuning inductance associated with the oscillator 26. rlhis will cause generated frequency of the oscillator to change from the marking frequency f1 to the spacing frequency f2. The operation of relay 15 against its spacing contact will disconnect condenser 20 from its shunt relation with condenser 19. Likewise the operation of relay 14 against its spacing contact will disconnect condenser 17 from its shunt relation with condenser 18.V This will leave only condensers 18 and V19 associated with transformer 32 and will thus constitute a pair of tuned circuits adapted to transmit the spacing frequency f2 instead of the marking frequency f1. Thus, while key 22 is held open, the spacing frequency f2 generated by oscillator 26 passes through amplifier 25, transformer 32 and through transformer 2 to the distant point over carrier line 1. The subsequent closure of key 22 will reoperate relays 14 to 16, inclusive, against their respective marking contacts thus reconnecting condensers 17, 20 and 26 in their respective circuits to again transmit the marking frequency f1 to the distant point.

Considering the operation of the receiving circuit, normally a marking frequencyA f3 is transmitted from the distant point over carrier line 1 and through transformer 2 to the receiving circuit. For this condition, the receiving circuit is operated as shown on the drawing. Current flows from plus battery 36 over the lower conductor of loop 23, through telegraph sounder 21, back over the upper conductor of loop 23 through resistance 37 and through the marking contact of polar relay 6, through minus battery 35 to ground. This current'holds sounder 21 operated against its marking contact. Assuming the conventional direction of current flow, i. e., from plus to minus battery, current also flows from ground through resistance 38, holding winding 13, armature and marking contact of polar relay 6 and throughnegative battery 35 to ground. This current holds the armature of relay 6 vroperated against its marking contact. Current likewise flows from ground throughr resistance 39, through the windings of polar relays 3 to 5, inclusive, in series throughl the marking contact of relay 6 and through negative battery 35 to ground. The latter current likewise operates relays 3 to 5 against their ,respective markingl contacts M. y

With relay 3 operated against its marking contact, only condenser 8 is in series with the left winding of transformer 40. Likewise, with relay 4 operated'against its marking contact, only condenser 9 shunts the right winding of transformer 40.y vThese capacities associated with the inductances of the transformer windings constitute a pair ofl tuned circuits adapted to transmit the spac- ,ing frequency f, from the distant point.

Since, however, under present assumption, the marking frequency f3 is being transmitted from thev distant point, the received signaling current isrthus prevented,l by the tuned circuit referred to, from entering the receiving amplifier and detector 24.

Assume, now, that the spacing frequency f., is transmitted from theY distant point. This current upon arriving at the central oiiice flows from the carrier line 1, through transformerv 2 and through transformer 40 to the amplifier-detector 24 where the signal current is rectified. Assuming the conventional currentv ow, that is from plus to amazes 1ninus, the rectified current in fthe platecir `cuit of the detectorflows from the vdetector .over ,the plus lead, through the inarkingcon-y tact of vrelay 5, winding12 of relay'giand back over theminusl leadvr to the detector. `fllhis current-acting] through Winding ,12 of *relay (i is Vof v suliicient; magnitudev to operate the. armature ofrelayfAagainstQits spacing y c ont act, against the effect ofjthe holding currentthroughwinding 13.-`

. Asfsoonas ythe arma-tureof relay fis thus operated A against the z spacing .contact S, plus battery 34'is substituted for minus .battery 35. f .This reverses .the direction ot current flow vtelegraph Ysounder-21. l lThese lopposed bat- 4teries thus preyentl` further current flow lthrough'.-sounder, 21 vWhich is thereu-pon Opz ierated toits spacing contact bylneansfof :the Y l Y* for IWinding 11' inthe :plate,.circ uit .otde-` Y ,usualsprin'g tension.. 1.y

' The VpositiveA battery 4the.;spacing lcontact ofrelay 6 vfurthermore reyersesthedirection of current 'Howthrough thefwindingsofpolar relays 3, to'5, inclusive, causing all of .theseyrelays to bei operated f against' :their respective spacingcontacts S,

The'op'eration `o 'fjrelay g3 'connects condenser -7 ingparallelfwitlrcondenser .8, botho saidcon# densers now being-g infseriesfwith 'the4 left i Winding otransformer-10-w'Likewise the operation fout Vrelay 1 againstits Aspacingcone tact;` connects vcondenser 10 'inparallel with f condenser 9, both .of -saidLcondensers' now shuntingdth'e rightwinding oftransforrner 11,0..I I Thisehangesthe tuningfofthe trans-v formerg circuit *to receive the marking fre* .quencyf f3 vinstead of the 4spacing frequency:

fr. The resultisthatieven. though the spac 4ing frequency f41-continues'to be transmitted,

tector 2 4, since the `circuit `is now/prepared to receive `the next marking requney-- substitutes ftherefor Winding 11 .of )relay 6:V

` quencys transmitted from,,the'distantpoint and flows transformerZ-to the ceivl'l'r; Channel Quency f3, the receiyed signaling."cur-rent ilow. .through ,transformer 4;0 .into amplilierf.

Vc letectn 24; .y :Tliejrectiiiedicurrent thus flow@- ing inthe yplatecircuitthrough Winding vreitera#.es relai its marking-:eea:

34 l supplied Athrongh `rence Lof the interference. i

tact. M,'g.despite the holding current iloxx/ing through I Windingv ,13 which tends to hold the relay against its spacing contact. The operation. of relay; 6 againstits marking v Contact replaces plus battery 84 byminus batteryf- .Current from minus battery. again reverses the direction of'current flow. through Winding 13 and gthusfholdsthe armature ofrelay 6 againstits marking contactgmAlso current now. yflowsfrom pluskbattery 36 through the sounder 21 toninusbattery l35 and reoperates -thegsounderV against its )marking contact."

Andgagain; the. minus ybattery 3 5.' reverses ythe i direction of current iloW through polar .re-y lays 3; tov 5,` 1nclus1ve, operat1ng the saine against their ,r csi 3ect1ve amarki-ng contacts.

their vshunt relation"Withcondensers 8; and

9,` respectively, thus', Drepari ng=.the circuitffor reception.'oithenext succeeding spacing fre? quency 72. The operation offrelayfagainst 7- I i. t s.niarking contact :substitutes ,Winding i 12 tector 24:,1thus preparingvfongthe operation of polar'lrelay 6 against itsy spacing l'contact is receivedfI v I ofthe circuitoperati'on givenfabove'thatzthe' .nervgfdisconnects condensers 7, and. 10,fre'm Vwhen thenext succeedinggspacin-g frequency f current .in afhold ing Winding, ',Howeverf, thifs tei-ference', such-.asl lightning, operatesy the position in ,which .it was .prior' to v the occur;

`the@arrangerment described thatthe shaped of the .'Inarkingand spacing `yWaves j should operation as. contrasted withx the usual fopen- LIt ,Wil-Lbe 4urthernaorefnoted frorrr-mstudy l carrier-systeins..jv :The system'has'gthe further. advantage lpointedlout Yalooyegof@attenuating yIt should vbelfurther pointed out here -`that the marking and spacing frequenciesgat ally tiniesby a likel amount, since both frequencies 'are transmitted-from vthe distantendl over the carri'erlinef.` Thus, signalbiasfis eliminated.

generalAtherelneed bere'latively little sep@l A karatijonbetweenthe markingfand spacing vfre-y y quencies transrnittedfromf-a .giyenjfstatiom I ksince the :effQC-t @fsasmall amountof spacing current passing throughthefffiltersf 'at fthe rei vceivingfend'vv'fhile the flters-aregtunedftotrans# mit themarkingfnequencyiorfvieeversa; may

be compensated for by properly adjusting the current through the holding winding of the receiving relay 6. In fact, it is advantageous to have these frequencies as close-ly the same as practicable, since this will insure like attenuation of both frequencies in transmission over the carrier line and thus insure that no signal bias will be introduced.

What is claimed is:

1. Carrier current telegraph means comprising in combination, a telegraph line terminating at a certain point, transmitting means associated therewith at said point for applying to said line alternate pulses of two distinct `single frequency currents, and receiving means, including a single tuned circuit, associated with said line at said point for receiving alternate pulses of two distinct single frequency currents and being operative in one sense by current of one said frequency and operative in a different sense by current of the other frequency.

2. Carrier current telegraph means cornprising in combination, a telegraph line extending from a first to a second point, transmitting means associated with the line at the first point for applying to said line alternate pulses of two distinct single frequency currents, and receiving means, including a singie tuned circuit and polar relay means, associated with said line at the second point for receiving said transmitted pulses, said relay means being operated in one sense by current of one frequency and in a different 4sense by current of the other frequency.

3. Carrier current telegraph means comprising in combination. a telegraph line terminating at a central office. a source of oscillation at said office, a tuned circuit interposed between said line and the output of said oscillator, a subscribers loop extending from said office to asubscribers station, sending means associated with the loop thereat, relay means associated with the loop at the central office and operable in accordance with said sending means, means controlled by said relay means for causing said loscillatorto generate one frequency while said relay'means is operated in one sense, and a different frequencyV while operated in the other sense. means controlled by said relay means for adjusting said tuned circuit to transmit the particular frequency being generated by said oscillator at a given instant.` j j 4. In a carrier current telegraph system in combination, a tuned circuit for transmitting one of two frequencies, means for applying thereto current of such frequency, detector means associated therewith for rectifying the same, a receiving relay responsive to said rectified current and operable to a first position thereby, means effective upon such operation for adjusting'said tuned circuit to transmit the second frequency, and to prepare said receiving relay for operation to-a second position by rectified current, means for applying to the tuned circuit current of the second frequency to so operate the receiving relay, means effective upon such operation to adjust the tuned circuit to transmit current of the first frequency, and to prepare the receiving relay for operation to the first position by rectified current, means thereafter for applying current of the first frequency and then of the second frequency alternately to cause the cycle of operations described to be indefinitely repeated while such alternate pulses are applied, biasing means effective upon operation of thereceiving relay to a given position to hold the same thus operated until current of the succeeding frequency is applied, D. C. telegraph receiving means associated with said receiving relay and operable in accordance therewith.

5. In a carrier current telegraph system in combination, a tuned circuit Vfor transmitting one of two frequencies, means for applying thereto current of the transmitted frequency, detector means associated therewithV for rectifying said current, a receiving relay associated with the detector outputand operable to a first position by said rectified current, a circuit completed by such operation to operate additional relayV means to a first position, means controlled through the latter operation to adjust said tuned circuit to transmit the second frequency and to preparesaid receiving relay for operation to a second position by rectifiedV current, means to apply current of the second frequency to the tuned circuit for so operating the receiving relay, a circuit effective thereupon for operating said additional relay means toa second position, means controlled through the latter operation to ad-r just the tuned circuit to again transmit the first frequency and to prepare the receiving relay for operation to the first position, means thereafter to apply the first frequency and then the second frequency alternately to cause the cycle of operations described to be repeated indefinitely, biasing means associated with the receiving relay to hold the same operated in a given position until acted upon by rectified current of the succeeding frequency D; C. telegraph receiving meansV associated with said receiving relay and operable in accordance therewith.

6. In a carrier current telegraph system in combination, a tuned circuit for transmitting one of two predetermined frequencies, means for applying thereto current of said frequency detector, means associated therewith for rectifying the same, a polar relay, an operating winding therefor connected in said detector output for operating the armature thereof against a first contact by said rectified current, a potential source of predetermined polarity associated with said first contact, additional relay means associated with said armature and operable to a first position by 'said'Y potential source,- means with said additionalrelay means thus operatedfor'- adjustingj'said tuned'circuit'to transmitthe" second of said predetermined frequencies, and for reversingthe connection of said polar relay operating Winding, means for applying to said tunedvcircuit currentfof said secondv frequencywhereby` rectified; current in said reversed polarrelay winding operatesisaid armature against'a secondcontact, aj otential i source of polarity "oppositeto thaty rst men-k tioned l associated L With said 'secondffcokntact, g

whereby said additionalrelaymeans is loperated toa second position, Ameansfeifective upon such operation for ,adj usting said-tuned ycircuit fto transmit theffirst frequency and fora again reversing the connection ofsa'id'polar, relayl operating Winding, said receiving meansy vbeing operable thereafter' to repeat the cycle. of 4operations While'v currents of said-v first andl second frequencies are A alternately applied thereto, a holding :Winding for said v polar relay associatedvvitli the armature thereof responsive tof current fromr` either.` potential sourcetobia'sthe armature against either contact after 'contact/therewith, telegraph receiving means associated with said armature re-v sponsive uniquely'to each potential source.

fr' 7. 'A carrier` current telegraph system conrprising inf combination a `telegraph line 'extending=fromafirst to asecond oince, a sub scriberlsjstation Withtele'graphsendingmeans Y thereat'adjacent the first; oflice,a loop circuit" connecting the subscribers'station to said office,1relay^means lassociated with. the -loop at said oiiceresponsivetothe operation of said sending means', a source -of oscillation asso-j` ciated With'the telegraph line at the first office, meansgc'ontrolled by said yrelay means forV causing said oscillator' to transmit Sover saidv line alternate pulsesoftwo distinctsingle fre# quencyfsignaling.currents in accordance with the koperation of* said subscribersI sending means, a tuned circuit associated with said' line atthe'second office adjusted to'` transmitv said signaling frequencies received yfrom' theA first office, detectorr means connectedlt'oy said tuned circuit for rectifying said received ysigy naling current,l relay meansconnec'ted in said detector output operable to aiirst position bysaidrectifiedlcurrent upon lreception of said frequency, means effective `upon such-*operaf*v tion-foradjusting`the tuned circuit to trans- 1 t mit the second frequency and for Q prepa-ring I said relaymeans for operationtoafsecond positiony by rectified current uponre'ception of said. second` frequency, means 'effective upon such operation for adjusting 'saidltuned circuit'to again"k transmitthe first yfrequencyand for preparingsaid relay means for opera?l n tion to the first position, said meansat the second 'office being` adapted thereafter to repeatthecycle of operations describedwhile said alternate'frequencygpulses of current are received,A a subscribers loop:associated,1v with `v said', relay means at thesecond oicefandextending tofa'local subscribers station, tell` egraph receiving meansthereat connected to said loop `responsive-to the operation of said relayZ means' at the second oii'ce.-`

8.11 In' the'- artofjtelegraphy, the' method' of signaling, Which consists ,iny transmitting alternately' currents oftWo-distinctfrequenclesl A through acircuit resonant totcurrent only of that t 4frequency beingAL transmitted lat a given l inst'antg'and utilizing the current ofsaid-fre- 'quencytogcause the operation ofv telegraph n l signaling "means and to causethetuning of said: resonant 4circuit tov transmitcurrent of the other frequency.'v f 1 j n '9. Inithe art oftelegraphy,-themethodof signaling, which consists in.t-rans1nitting al'- Y ternately currents of tWo distinct frequencies through a circuit resonant to current only-of` i thatflfr'equency beingy transmitted at a gi'ven instant,`futilizing the currentl of' saidtIk freik quency f to causev the loperation of ktelegraph signa-ling means and to cause the tuning ofv said resonant circuit to Y transmit current of thetotherl frequency, and utilizing at another given instant current of said' otherk frequency to cause the operation of telegraph signaling means vand to cause the tuning ofV said reso'-l nant circuit to transmit current of the'firstV frequency'- p. 10,` The method "of carrier `currenty ytelegra'p'hy, V'Whichconsists-in transmitting alterthat yfrequency being transmitted ata `given instant,:-utilizing the current ofsaid fre-A 'natelyb currents of :two distinct'frequenciesf through aV circuit-resonant to current onl of quencyfor operating polar relay means in vaj certain sense to cause the operation of1tele-' graph signal means and to cause the tuninglof'l saidfjres'onant' circuitto transmit current of the other frequencies, and utilizingat anotherk given instant current of said other frequency for operating polar relay means in the'oppositesense to cause the'operation' oftelegraph'f signal means and to cause thevtuningffof said resonant circuit'to transmit current ofthe its marking' contact' and to cause thetuning of A said fr resonant Y circuitr to another 'distinct 11; The method of`=carrier current teleg-ffl frequency, transmittingl an alternatingfcur- Y rent pulse'of-'said other distinct {frequency Vfor a spacing-signal, rectifying said spacing sig.- nal pulse, and `utilizing saidA rectifiedspacing` signal pulse 1 to causethe operation' of'said vtelegraph @receiver to its spacing contact and:

tojcause thetuningfof said resonant cifrcuitto' thef:frequencyJ of saidf marking signal pulse:` y :4

12. In a telegraph system comprising a receiver and a receiving circuit including a resonant circuit, a rectifier and relay means, the method of signaling, Which consists in transmitting an alternating current pulse 0f a predetermined frequency through said resonant circuit, rectifying said pulse, operating saidrelay means with said rectiiedpulse to operate said receiver to its marking contact and to tune said resonant circuit to another distinct frequency, transmitting anv alternateing .current pulse of said other distinct frequency through said resonant circuit, rectifying the pulse of said other frequency, and operating said relay means With the rectilied` pulse of said other frequency to operate said receiver to its spacing contact and to retune said resonant circuit to the firstfrequency.

i3. In a telegraph system comprising a reci er and a receiving circuit including a reciiiicr, polar relay means and a resonant circuit tuned to transn'iit current only of a predetermined frequency, the method of signaling,`which consists in transmitting an alternating current pulse of said predetermined frequency through said tuned circuit, rectifying said pulse, utilizing said rectified pulse for operating said polar relay means in a predetermined sense to cause the operation of said receiver to its marking contact and to tune said resonant circuit to a current of another distinct frequency, transmitting an alternating current pulse of said other distinct frequency through said tuned circuit, rectifying the pulse of said other frequency, and utilizing said latter rectified pulse for Operating said polar relay means in the opposite sense to cause the operation of said receiver to its spacing contact and to tune said resonant circuit to a current of said first predetermined frequency.

14. In combination, a source of oscillation, a tuned circuit associated with the output of said oscillator, a circuit including relay means, a key adapted to control the operation of said relay means, means controlled by said relay means for causing said oscillator to generate one frequency While said key is operated in one position and a different frequency while said key is operated in another position, and means controlled by said relay means for adjusting said tuned circuit to transmit the particular frequency being generated by said oscillator at a given instant. 15. In a carrier current telegraph system includinga source of oscillation, polar relay means and a tuned circuit associated With the output of said oscillator, the method of signaling, which consists in operating said relay means in one sense to cause said oscillator to generate one frequency and simultaneously to adjust said tuned circuit to transmit said frequency, and operating said relay means in the other sense to cause said oscillator to generate another distinct frequency and simultaneously to adjust said tuned circuit to transmit said other frequency.

16. In a carrier current telegraph system, in combination, a telegraph circuit extending from a first point to a second point, a sending key and a source of oscillation associated With said telegraph circuit at said first point, a tuned circuit associated With the output of said oscillator, means controlled by the operation of said sending key forcausing said` oscillator to generate alternately pulses of two distinct frequencies and for adjusting said tuned circuit to transmit pulses of the frequency being generatedby said oscillator at a given instant, a circuit at said second point resonant only to the frequency being transmitted at a given instant, and means associated With said resonant circuit for causing thev operation of telegraph receiving means at said second point and for causing said resonant circuit to beadj usted to transmit pulses of said other frequency at another given instant.

17. The method of controlling the operation of polar relays With alternating currents of two frequencies, Which consists in transmitting current of one frequency through a tuned circuit resonant only to the frequency being transmitted, rectifying said current, transmitting said rectified current through a first Winding of said polar relay to operate said relay in a predetermined sense, causing the operation of said relayin said predetermined sense to render said tuned circuit resonant only to another frequency and to connect a second Winding of said polar relay in place of` said first Winding, transmitting current of said other frequency through said tuned circuit, rectifying said current, transmitting said rectified current through said second Winding to operate said polar relay in the opposite sense and causing the operation of said relay in said opposite sense to again render said tuned circuit resonant only to said first frequency and to reconnect said rst Winding of said polar relay in place' of said second Winding;

18. Means for controlling polar relays With alternating current of tWo frequencies, comprising means for generating alternately currents of tWo distinct frequencies, a tuned circuit associated with said generating means and resonant only to one of said frequencies at a given instant, a rectifier adapted to rectify the current transmitted through said tuned circuit, a polar relay including a rst Winding and a second Winding, means for transmitting said rectified current through said first Winding at a given instant to operate said relay in a predetermined sense, means controlled by the operation of said relay in said predetermined sense for adjusting said tuned circuit to transmit only the other of said frequenciesmeans controlled by August, 1928. n v VAUGHN P. THOR?. 

